Nok Mini SF34 at Udon Thani on Oct 6th 2013, taxiway excursion, nose gear collapsed
Last Update: September 12, 2022 / 14:45:42 GMT/Zulu time
The airline initially reported a glitch with an engine, later said however that the aircraft had already slowed to taxi speed and was taxiing along the taxiways when the aircraft's hydraulic nose wheel steering malfunctioned.
The airport reported the aircraft had landed normally on runway 30 and had vacated the runway onto taxiway C but failed to turn right onto taxiway N towards the terminal and came to a stop on soft ground off the taxiway.
On Apr 28th 2020 Sweden's Haverikommission (SHK) reported: "Despite repeated requests from SHK for several years, SHK has not yet received a draft final report. It can no longer be considered justified to still keep the case open at SHK. Should a draft report nevertheless arrive in the future, there is no obstacle to reopening the case." The SHK's brief abstract reads: "In connection with taxiing after landing with a fault on the hydraulic system, the aircraft (registration designation HS-GBG) landed in a muddy area off the taxiway and collided with a concrete foundation. The aircraft suffered significant damage. There were no injuries."
On Sep 12th 2022 the final report surfaced on an inofficial resource of Thailand's Ministry of Transport concluding the probable causes of the accident were:
The Aircraft Accident Investigation Committee considered that the accident is caused by both pilots did not follow the Abnormal Procedures in the Aircraft Operations Manual to solve the hydraulic system malfunction when the single chime came on several times, during the flight. Until commencing the holding, the pilots performed the Abnormal Procedures on Hydraulic Fluid Lost but not complete and correct. After landing, the pilot taxied the aircraft from the runway to the apron, during that time, the nose landing gear steering and the brake systems were inoperative because hydraulic pressure in two accumulators decreased until unable to supply to those two systems. Subsequently, the pilots were unable to control the direction and stop the aircraft. The aircraft veered off the taxiway and hit the concrete base of the aerodrome information sign. The aircraft was destroyed.
The Accident Investigation Commission reported: "After the accident, the Department of Civil Aviation ordered the pilot and the co-pilot to have Pilot Proficiency Check in flight simulator. They had passed the test conducted by the Department of Civil Aviation." and provided a brief abstract:
During flight, the hydraulic system malfunctioned. The pilots did not completely and correctly follow the Abnormal Procedures in the Aircraft Operations Manual. After landing on runway 30 at Udon Thani Airport, while the aircraft was taxiing on taxiway C, passing the intersection of taxiways C and N, the aircraft veered to the left side of the taxiway to the ground and hit the concrete base of the aerodrome information sign before it came to stop. The aircraft was destroyed.
In the narrative of the sequence of events the Commission wrote:
At 07:23:44 hours, after starting both engines, a single chime came on for the first time. The pilot expressed doubtfulness of the malfunction occurred. Before continuing to operate the flight, the pilot assigned the co-pilot to be a Pilot Flying responsible for taking off the aircraft.
At 07:30:18 hours, after the aircraft took off from runway 36, while the landing gear was in retraction process, the single chime came on for the second time. Both pilot and co-pilot discussed that the warning was caused from losing of the hydraulic system. However, the pilot considered disregarding this problem and continuing to operate the flight because the landing gear was in down position.
At 07:35:31 hours, the single chime came on for the third time. The pilot was curious about the illuminated warning light that had never happened before.
At 08:37:14 hours, Udon Thani Approach cleared the aircraft to descend to 3,500 feet, perform Instrument Landing System (ILS) approach runway 30 and report over Intermediate Fix (IF).
At 08:43:05 hours, the pilot reported over IF. The pilot then contacted Udon Thani Tower, as instructed by Udon Thani Approach, to request ILS approach runway 30.
At 08:43:41 hours, the single chime came on for the fourth time. The pilot suspected about the repeated hydraulic system malfunctions. Both pilots were discussing on how to fix insufficient hydraulic pressure by selecting the hydraulic pump control switch to OVRD (Override) position to extend the flaps and the landing gear. The pilot said there was no time to perform the Normal Operating Procedures indicated in the Aircraft Operations Manual (AOM), and mentioned that hydraulic pressure was at zero.
At 08:45:35 hours, while the aircraft was at five nautical miles from runway 30, the pilot contacted the Air Traffic Controller (ATC) to report the landing gear problem and request to hold at altitude 2,500 feet. While climbing to the requested altitude, the pilot took control from the co-pilot. The co-pilot was trying to retract the landing gear and the flaps but the retraction could not be done. At that time, the autopilot disengage warning came on twice.
At 08:48:01 hours, both pilots followed the Abnormal Checklist, Hydraulic Light ON, Emergency Pressure Normal and Main Pressure Low in the AOM, by using hand pump (auxiliary system). They were able to extend the landing gear and retract the flaps.
At 08:51:08 hours, the pilot informed the ATC that the aircraft was reaching altitude 3,500 feet. At that time, another autopilot disengage warning came on. The ATC informed the pilot that there was another aircraft requesting to take off from runway 30, the pilot requested that mentioned aircraft to wait. The ATC asked the pilot whether the aircraft required any assistance, the pilot denied and requested to return to the Very High Frequency Omnidirectional Range (VOR). Both pilots discussed about the landing gear extension, the autopilot disengagement system and the insufficiency of hydraulic pressure.
At 08:52:12 hours, the single chime periodically came on for three consecutive times. At that time, the aircraft was flying over the VOR. The co-pilot requested the ATC to hold at altitude 3,500 feet over the VOR. Both pilots were discussing about the problem of the flap and the landing gear extension. The co-pilot asked the pilot whether the green light should illuminate when the landing gear was extended. After that, they were able to extend the flaps.
At 08:55:00 hours, the pilot contacted the ATC to request outbound to 4 DME position in order to go to IF to perform ILS approach and also informed the ATC that the landing gear and the flaps had already been extended. The ATC asked whether the aircraft need any assistance, the pilot only requested the fuel truck for refueling the aircraft upon arrival. The co-pilot asked the pilot whether the green light would not illuminate when the landing gear was extended. Both of them made a conclusion that this problem had been occurred since take-off phase.
At 09:02:52 hours, both pilots performed landing procedure. The pilot gave control to the co-pilot to control the aircraft and ordered to use full reverse thrust due to the brake may be inoperative.
At 09:05:05 hours, while the aircraft was vacating from the runway to taxiway C, the pilot was controlling the aircraft. The co-pilot suggested twice to use hand pump (auxiliary system).
After that, the co-pilot suggested to stop the aircraft and request for the aircraft towing tractor to tow the aircraft.
At 09:06:14 hours, the co-pilot reported the ATC about the hydraulic system malfunction and the inoperative brake, and added that the aircraft might need to stop on taxiway N. The pilot said the aircraft had to stop at the present position because it could not continue. The co-pilot said that the aircraft was still moving. Then, the pilot asked whether the engines should be shut down. The co-pilot suggested turning the aircraft first. The pilot replied that he was unable to steer the aircraft and ordered to shut down the engines.
At 09:07:04 hours, a triple chime came on, the co-pilot said that the aircraft hit something. The pilot gave the order to shut down the engines. The co-pilot read back the order to shut down the engines and said parking brake.
The aircraft was destroyed from this occurrence. All crew and passengers were safe. The accident occurred at 09:07 hours.
The captain (64, ATPL, 30,506 hours total, 662 hours on type) was assisted by a first officer (36, CPL, 2,637 hours total, 148 hours on type).
The Commission also reported that the DFDR malfunctioned and did not record any data of the accident flight.
The Commission thus analysed:
Refer to the readout data from the CVR, since the pilots started the engines in the apron at Chiang Mai International Airport until the aircraft landed at Udon Thani Airport, the single chime aural warning came on for seven times. The related aural warnings and the flight operation actions of both pilots were as follows:
- The first single chime came on, (at 19 minutes 10 seconds after the pilot turned on the aircraft main electrical switch in order to start the engines) after starting both engines, both pilots expressed doubtfulness of the malfunction occurred. However, both of them still performed their duties and the pilot assigned the co-pilot to control and takeoff the aircraft, without solving above-mentioned malfunction. If the warning had come on because of hydraulic system malfunction, both pilots would have followed the Abnormal Procedures in the AOM, the Quick Reference Handbook (QRH) as well as the Minimum Equipment List (MEL). In this occurrence, the pilots should not have continued the flight.
- The second single chime came on 6 minutes 32 seconds after the first chime during the landing gear retraction process. The conversations between both pilots after this warning indicated the hydraulic loss but the pilot disregarded this problem as the landing gear remained extended and continued operating the flight. At 5 minutes 8 seconds later, the third single chime came on. The pilot was curious about the illuminated warning light which had never happened before, but they still performed their duties as usual without taking action to solve the problem.
From the second warning, both pilots considered hydraulic system malfunction. However, they did not follow the Abnormal Procedures in the AOM, and had not planned for preparation in advance in the event of aircraft hydraulic system malfunction, especially during landing when the pilots had high workload such as flying the aircraft, communicating and rectifying the malfunction of hydraulic system required for many aircraft systems.
- The fourth single chime came on 1 hour 8 minutes 17 seconds after the third warning, while the aircraft was approaching Udon Thani Airport. After that, the pilot expressed doubt about repeated hydraulic system malfunctions. Both pilots discussed how to fix insufficient hydraulic pressure by selecting hydraulic pump control switch to OVRD (Override) position to extend the flaps and the landing gear. Afterwards, the pilot said that they had no sufficient time to perform the Normal Procedures indicated in the AOM, and mentioned that hydraulic pressure was zero.
This warning occurred when the aircraft was approaching Udon Thani Airport. Both pilots had not yet followed the Abnormal Procedures indicated in the AOM though they considered that the problem was caused from hydraulic system malfunction. But the pilot had solved the problem on a trial and error basis which resulted in problem still remain unsolved. The pilot could not land the aircraft and had to request holding in order to deal with the problem.
- During holding, the pilot and the co-pilot tried to solve the problem of the flaps and the landing gear extension by checking hydraulic pressure in the brake accumulator which was found in normal level, but low pressure in the main accumulator was discovered. The pilots then performed the Abnormal Procedures in the QRH in order to retract the flaps and extend the landing gear. The co-pilot was responsible for reading the Abnormal Checklist when Hydraulic Light ON in the AOM but he did not completely and correctly read and follow the Emergency Pressure Normal and Main Pressure Low which advises increasing hydraulic pressure in the main accumulator before landing. If either Hydraulic quantity or Hydraulic main pressure or both low, use the HAND PUMP to assure nose wheel steering function during roll out. If hydraulic pressure is lost, nose wheel steering will be inoperative. At that moment, the fifth single chime came on, 8 minutes after the fourth chime, the sixth single chime came on 16 seconds later. Then the seventh single chime came on after 3 seconds, before successfully extending the flaps and the landing gear, and the pilot requested for landing clearance. At approximately 1 minute 35 seconds prior to touching down, the pilot gave control to the co-pilot and the co-pilot was able to land the aircraft.
Refer to the SAAB 340B AOM, the warning annunciator system monitors various parameters in the aircraft that matters to the aircraft safety (There are entirely 31 warning annunciator systems installed.) when one or more aircraft systems malfunction (below or exceed limits). The warning annunciator system would issue warning or caution according to the severities of indicated defects. There are eight warning systems, consist of red light and the triple chime, or, 23 caution systems with amber light and the hydraulic caution system is included in this type of caution.
Regarding the CVR data of the accident flight, the single chime came on for seven times. Also, the conversations between the pilots, after each warning, mentioned about hydraulic system malfunction. During holding, after the fourth single chime came on, the pilots solved hydraulic system malfunction by following the Abnormal Procedures in the AOM. All seven single chime warnings might be a caution of hydraulic system.
After the co-pilot landed the aircraft, the pilot taxied the aircraft from the runway to taxiway C as advised by the ground controller. At that moment, the co-pilot suggested the pilot twice to use auxiliary system by selecting hand pump first. Then the co-pilot suggested to park and tow the aircraft. But, the pilot insisted on waiting. The co-pilot informed the controller about the problem that caused the aircraft to stop at the present position and the controller acknowledged. During that time, the aircraft stopped for a moment, then moved backwards before coming to stop and moved forwards again until passing the intersection between taxiways C and N. The aircraft veered to the left side of the taxiway, went across taxiway N to the ground and hit the concrete base of the aerodrome information sign before it came to stop between taxiways N and K.
The pilot taxied the aircraft from the runway to the apron which did not comply with the After Landing procedure indicated in the Abnormal Procedures in the AOM that specified caution “Do not taxi into ramp area or crowded areas with a faulty hydraulic pump”, and note “During taxiing with a nonfunctional hydraulic pump, use nose wheel steering and brakes with great care. The functions will be abruptly lost”.
The post-accident examination showed that Hand Pump Selector was in FLAPS LDG GR (Main Accumulator) position. In this case the pilots should have followed the Abnormal Checklist in the AOM by setting Hand Pump Selector in OUTBD BK ACC (Outboard Brake Accumulator) or INBD BK ACC (Inboard Brake Accumulator) position in order to increase hydraulic pressure in the selected accumulators to be in appropriate level and able to supply hydraulic to the brake system.
During flight, the ATC repeatedly asked whether the aircraft needed any assistance from the ground services. The pilot only requested the fuel truck for refueling the aircraft. If the pilots followed the After Landing procedure indicated in the Abnormal Procedures in the AOM that “Do not taxi into ramp area or crowded areas with a faulty hydraulic pump”, after landing the aircraft, both pilots should have stopped the aircraft on the runway and requested for the aircraft towing tractor to tow the aircraft from the runway to the apron.
During the communications in many phases of the accident flight, both pilots mentioned about hydraulic system when the single chime came on, which showed that they had insufficient knowledge and understanding on the operation of hydraulic system, especially, the landing gear and the brake operations, the compliance with the checklist as well as the operating instructions regarding own duties and responsibilities and the relevance of such duties to the overall operation. Refer to the reviewing of the maintenance records of the aircraft, the engines, the hydraulic system and other systems, there was no deferred detect found.
Refer to the readout data from the CVR at 09:07:04 hours, the triple chime came on and the co-pilot said the aircraft hit something. It is assumed that the aircraft hit the concrete base of the aerodrome information sign causing the nose landing gear to collapse and the electrical wiring in the forward section of the aircraft was damaged which made electrical systems inoperative.
VTUD 060300Z 10006KT 5000 HZ NSC 29/19 Q1012 A2990 INFO E RWY30
VTUD 060200Z 04002KT 4000 HZ NSC 27/21 Q1012 A2991 INFO D RWY30
VTUD 060100Z 05002KT 3000 BR NSC 26/21 Q1012 A2990 INFO C RWY30
VTUD 060000Z 00000KT 2500 BR NSC 24/21 Q1012 A2989 INFO B RWY30
VTUD 052300Z 00000KT 2000 BR FEW012 OVC080 22/21 Q1011 A2986 INFO A RWY30
This article is published under license from Avherald.com. © of text by Avherald.com.
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